Interferometric measuring device

ABSTRACT

An interferometric measuring device for measuring a surface or an interface of an object (O) to be measured is described. The effect of vibration on the measurement result is suppressed by providing a connecting unit (VE) by which at least one section of the measuring device assigned to the object (O) to be measured is mechanically rigidly and detachably connectable directly to the object (O) to be measured.

[0001] The present invention relates to an interferometric measuring device for measuring a surface or an interface of an object to be measured.

BACKGROUND INFORMATION

[0002] An interferometric measuring device of this type is described in German Patent 197 21 843 C1, where this known interferometric measuring device is designed as a white light interferometer measuring device having a section designed as a modulation interferometer and a section designed as an imaging interferometer assigned to the object to be measured. The imaging interferometer is designed so that measurements may be performed even in tight spaces. It is proposed here that a first partial beam be further divided into a reference partial beam and at least one measuring partial beam, another beam splitter and a reference mirror being situated in a common measuring probe. Such a measuring probe may be inserted into hollow spaces. For example, if vibration occurs during a measurement in a manufacturing process, the measurement result may be inaccurate due to vibration of the measuring device and/or the object to be measured.

[0003] The principles of white-light interferometry or short-coherence interferometry are described, for example, by P. de Groot, L. Deck, “Surface profiling by analysis of white-light interferograms in the spatial frequency domain,” J. Mod. Opt., Vol. 42, No. 2, 389-401, 1995, and T. Maack, G. Notni, W. Schreiber, W. D. Prenzel, “Endoskopisches 3D-Formmesssystem [Endoscopic 3D Mold Measuring System]” in Jahrbuch für Optik und Feinmechanik [Yearbook for Optics and Precision Mechanics], W. D. Prenzel, ed., Verlag Schiele & Schoen, Berlin 231-240, 1998.

[0004] Furthermore, the design of an interferometric measuring device is also described in German Patent 197 21 843 C2. In the case of this known measuring device, a radiation generating unit, e.g., an LED or a superluminescence diode, emits a short-coherent radiation which is split by a beam splitter into a first partial beam, which is sent via an object light, and a second partial beam which is sent via a reference light path. The reference light path is varied periodically by two deflector elements and a stationary diffraction grating situated behind it by triggering the deflector elements to scan the object surface in the depth direction. If the object light path and the reference light path correspond, the result is a maximum interference contrast, which is detected by an analyzer device downstream from the photodetector device.

[0005] In the case of another white-light interferometer measuring device described in German Patent Application 41 08 944 A1, a moving mirror is used to change the light path in the reference beam path. Even with these methods, the possibility that vibration of the measuring device and/or the object to be measured will affect the measurement result cannot be completely ruled out.

[0006] The object of the present invention is to provide an interferometric measuring device of the type mentioned in the preamble with which the effects of vibration on the measuring device and/or the object to be measured on the measurement result can be completely ruled out.

ADVANTAGES OF THE INVENTION

[0007] This object is achieved with the features of claim 1 by providing a connecting unit by which at least one section of the measuring device assigned to the object to be measured is mechanically rigidly and detachably connectable directly to the object to be measured.

[0008] Due to the mechanically rigid connection between the object to be measured and the interferometric measuring device, any movement during vibration is transferred equally to the measuring device and the object to be measured, so it does not lead to distortion of the measurement result. A mechanical adaptation between the object to be measured and the measuring device is established through the connecting unit for the measurement operation. Thereafter, the connection may be released and a mechanically rigid link established to the downstream object to be measured, e.g., to perform a quality check in mass production.

[0009] Various possible embodiments include the connecting unit being formed by a plug connection, a clamp connection, a chucking connection, a catch connection, a clip connection, or a screw connection.

[0010] In principle, it is possible to use these measures for vibration compensation in various interferometric measuring devices, e.g., in classic interferometry and in 2-wavelength interferometry with a corresponding design for the data acquisition.

[0011] In an advantageous embodiment of the interferometric measuring device, it is designed as a white-light interferometer measuring device having an object light path and a reference light path, and at least the reflecting reference plane is connectable to the object to be measured via the connecting unit.

[0012] A compact design is achieved by the fact that the object light path and the reference light path form a rigid interconnected unit having an object arm and a reference arm.

[0013] If the white-light interferometer has a modulation interferometer and an imaging interferometer assigned to the object, this yields a simplified operation, because the modulation interferometer, which is usually more complex, together with the device for altering the light path, is separate from the imaging interferometer, which is assigned to the object to be measured and has a simple and rugged design accordingly and may be adapted to the particular measurement task.

[0014] In detail, a design which is advantageous for handling is characterized, for example, by a short-coherent radiation-emitting radiation generating unit, a beam splitter for forming a first partial beam and a second partial beam, for the measurement the optical path length of the first partial beam being varied relative to the optical path length of the second partial beam, the first partial beam formed by the beam splitter being directed first to a stationary first mirror over a first arm, while the second partial beam is directed to the reflecting element via a second arm, the optical path difference between the first arm and the second arm being greater than the coherence length of the radiation, the radiation, coming from the first mirror and the reflecting element and being jointly forwarded, is directed in part to the object to be measured via an object light path and in part to a reference mirror via the reference light path using an additional beam-splitter, the reference mirror being situated at such a distance from the object to be measured that the path difference between the first mirror and the reflecting element is eliminated, and the radiation striking the reference mirror and the radiation directed to the object to be measured being reflected, superimposed and detected by a photodetector device having an image pickup.

[0015] A favorable design for handling includes the reference light path being formed in a separate reference office or in an optical probe through which the radiation directed to the object to be measured is also directed, the reference mirror reflecting the part of the radiation belonging to the reference light path and allowing the part of the radiation belonging to the object light path to pass through.

[0016] A simple measurement of different measurement surfaces is made possible by the fact that at least one optical element and/or the elements shaping the wave front of the radiation are situated in the object light path.

[0017] If an optical probe having an optical arrangement is provided in the object light path for generating at least one intermediate optical image, this permits a high lateral resolution of the measurement surface, as explained in greater detail in German Patent Application 100 47 495, which was not published previously.

[0018] Favorable embodiments include the fact that the at least one intermediate image is generated in the object light path, and both the radiation leading to the object to be measured and the radiation coming back from it pass through the optical probe.

[0019] A favorable design is also derived due to the fact that the reference mirror is provided on a flat plate or a prism.

[0020] It is advantageous for handling that fiber optics are situated between the beam splitter and the additional beam splitter.

[0021] For the design and operation, other advantages may be achieved by providing an endoscope in the object light path for generating the intermediate image.

[0022] Additional possible advantageous embodiments include providing panorama optics or superpositioning optics in the object light path and having multiple reference planes for scanning different measurement surfaces.

DRAWING

[0023] The present invention is explained in greater detail below on the basis of exemplary embodiments with reference to the drawing.

[0024]FIG. 1 shows a first exemplary embodiment of an interferometric measuring device having a modulation interferometer and an imaging interferometer rigidly connected to an object;

[0025]FIG. 2 shows another exemplary embodiment in which the imaging interferometer has a common-path design and is rigidly connected to the object; and

[0026]FIG. 3 shows another design of the interferometric measuring device having a common reference light path and measurement light path (common-path design).

EXEMPLARY EMBODIMENT

[0027]FIG. 1 shows an interferometric measuring device having a modulation interferometer MI at a distance from an object O to be measured and an imaging interferometer AI assigned to object O. Modulation interferometer MI has a radiation generating unit SLD which emits a short-coherent, i.e., broadband radiation, e.g., an LED or a superluminescence diode whose radiation is split by a beam splitter ST1 into a first partial beam T1 of a first arm and a second partial beam T2 of a second arm. This design corresponds to a Michelson interferometer. In the second arm, second partial beam T2 is reflected by a reference plane in the form of a reference mirror RSP1, the second arm being varied periodically, e.g., by moving reference mirror RSP1 or by acousto-optic deflectors, such as those described in the above-mentioned German Patent 197 21 842 C2. If the change in the light path is performed with two acousto-optic deflectors, a mechanically moving reflecting element is unnecessary, and a stationary element, in particular a grating, may be used instead.

[0028] The optical path difference between the arms thus formed is greater than the coherence length of the radiation generated by radiation generating unit SLD. From the two mirrors SP1 and RSP1, the reflected radiation is sent to the imaging interferometer via beam splitter ST1, fiber optics LF, and another beam splitter ST2.

[0029] An object light path having an optical probe OS and a reference light path having another reference mirror RSP2 are formed in imaging interferometer AI, which is also designed as a Michelson interferometer.

[0030] In the object light path, radiation is input into optical probe OS, so that radiation illuminates the surface to be measured of an object O to be measured. Through optical probe OS, the object surface is imaged on a photodetector device in the form of an image converter and/or image sensor BS, e.g., a CCD camera, via one or more intermediate images ZB. The reference wave of the second partial beam of the reference light path is superimposed on the image of object O to be measured on image sensor BS. A high interference contrast occurs in the image of object O to be measured when a path difference between the reference light path and the measurement light path is less than the coherence length.

[0031] The measurement principle is based on white-light interferometry (short-coherence interferometry), as described in greater detail in the publications cited in the preamble. The length of the reference light path is varied by moving mirror RSP1 in the second arm of modulation interferometer MI for scanning in the depth direction of the surface to be measured over the entire measurement range, the length of the reference light path at which the highest interference contrast occurs being detected for each measurement point. The intermediate images make it possible to image the surface of the object to be measured with a high lateral resolution over a distance which is great in comparison with the diameter of the imaging optics. Optical probe OS resembles an endoscope or a borescope, but the illumination and the return of the radiation coming from the surface to be measured are accomplished with the same optical arrangement using at least one intermediate image. FIG. 1 shows a few lenses L schematically as additional imaging elements. The actual intermediate images are generated in optical probe OS.

[0032] For applications which require a precise compensation of the influence of the imaging lenses of optical probe OS, the same optical probe may also be integrated into the reference light path between beam splitter ST2 and reference mirror RSP2 as into the object light path between beam splitter ST2 and object O to be measured.

[0033] The interferometric measuring device may be implemented in a modified design according to FIGS. 2 and 3, namely imaging interferometer AI and an arrangement having a common reference arm and measurement arm (common-path arrangement). The interferometric measuring device is in turn illuminated with a short-coherent (broadband) radiation generating unit. Beam splitter ST1 splits the light into first partial beam T1 and second partial beam T2, first partial beam T1 striking first stationary mirror SP1 and second partial beam T2 striking reflecting element RSP1 in the form of the reference mirror.

[0034] The optical path difference between the arms thus formed is in turn greater than the coherence length of the radiation generated by radiation generating unit SLD. From the two mirrors SP1 and RSP1, the reflected radiation is sent to optical probe OS via beam splitter ST1 and another beam splitter ST2. The particular feature of this design is that a reference mirror RSP2 is situated in the object light path and/or in optical probe OS itself.

[0035] A portion of the radiation is reflected on this reference mirror RSP2, while the other portion of the radiation illuminates the surface to be measured. Reference mirror RSP2 may be applied to a flat plate or to a prism. By using a prism, it is possible to adapt the wave front of the radiation illuminating the object surface, i.e., the object wave, to the geometry (e.g., the inclination) of the surface to be measured. Object O to be measured is in turn imaged on image sensor BS via one or more intermediate images by using optical probe OS and the reference wave is superimposed on it. To obtain height information, reflecting element RSP1 is moved over the measurement area, or the light path is varied as described in German Patent 197 21 842 C2 cited above. A high interference contrast occurs in the image of object O to be measured when the path difference between stationary mirror SP1 and reflecting element RSP1, i.e., the light paths of the two arms, is exactly equal to the optical path difference between reference mirror RSP2 and object O to be measured. Known methods of detecting the highest interference contrast in each pixel are used to obtain the 3D height profile. This design has the advantage that the object wave and the reference wave pass through virtually identical optics, so that aberrations are mostly compensated. Furthermore, this arrangement is even more rugged with respect to mechanical vibration.

[0036] For even simpler handling of the measuring device, the radiation of beam splitter ST1 may also be transmitted via fiber optics LF to the other beam splitter ST1, as illustrated in FIG. 2. As an alternative, a free-beam design may also be selected.

[0037] With all the designs described here, imaging interferometer A is designed as a rigid mechanical unit, at least in the section of the object light path and the reference light path, and during the measurement this mechanical unit is rigidly and detachably connected directly to object O to be measured by a connecting unit VE in a connecting area VB. For example, a screw connection having a union nut, a screw section of imaging interferometer AI which can be screwed into a threaded hole in object O to be measured, a plug connection, a catch or clip connection or a clamp or chucking connection is provided as the connecting unit. If a common-path design is selected, the object light path, i.e., the object arm, and the reference light path, i.e., the reference arm, may be introduced jointly into object O to be measured. 

What is claimed is:
 1. An interferometric measuring device for measuring a surface or an interface of an object (O) to be measured, wherein a connecting unit (VE) is provided by which at least one section of the measuring device assigned to the object (O) to be measured is directly connectable to the object (O) to be measured in a mechanically rigid and detachable manner.
 2. The measuring device as recited in claim 1, wherein the connecting unit (VE) is formed by a plug connection, a clamp connection, a chucking connection, a catch connection, a clip connection, or a screw connection.
 3. The measuring device as recited in claim 1 or 2, wherein it is designed as a white-light interferometer measuring device having an object light path and a reference light path, and at least the reflecting reference plane (RSP2) is connectable to the object (O) to be measured using the connecting unit (VE).
 4. The measuring device as recited in claim 3, wherein the object light path and the reference light path form a rigid, interconnected unit made up of an object arm and a reference arm.
 5. The measuring device as recited in claim 3 or 4, wherein the white-light interferometer has a modulation interferometer (MI) and an imaging interferometer (AI) assigned to the object (O).
 6. The measuring device as recited in one of the preceding claims, characterized by a short-coherent, radiation-emitting radiation generating unit (SLD), a beam splitter (ST1) for forming a first partial beam and a second partial beam (T1, T2); for the measurement, the optical path length of the first partial beam (T1) is varied relative to the optical path length of the second partial beam (T2); the first partial beam (T1) formed by the beam splitter (ST1) being directed first to a stationary, first mirror (SP1), over a first arm, while the second partial beam (T2) is directed to the reflecting element (RSP1) over a second arm; the optical path difference between the first arm and the second arm being greater than the coherence length of the radiation; the radiation that comes from the first mirror (SP1) and the reflecting element (RSP1) and is transmitted jointly being directed in part to the object (O) to be measured, via the object light path, using another beam splitter (ST2), and in part to a reference mirror (RSP2) via the reference light path; the reference mirror (RSP2) being situated at such a distance from the object (O) to be measured that the path difference between the first mirror (SP1) and the reflecting element (RSP1) is eliminated; and the radiation striking the reference mirror (RSP2) and the radiation directed to the object (O) to be measured being reflected, superimposed, and detected by a photodetector device having an image pickup (BS).
 7. The measuring device as recited in claim 6, wherein the reference light path is formed in a separate reference arm or in an optical probe (OS), through which the radiation directed to the object (O) to be measured is also directed, the reference mirror (RSP2) reflecting the part of the radiation belonging to the reference light path and allowing the part of the radiation belonging to the object light path to pass through (common-path arrangement).
 8. The measuring device as recited in one of claims 3 through 7, wherein at least one optical element and/or the elements shaping the wave front of the radiation are situated in the object light path.
 9. The measuring device as recited in one of claims 3 through 8, wherein an optical probe (OS) having an optical arrangement is provided in the object light path for generating at least one intermediate optical image.
 10. The measuring device as recited in claim 9, wherein the at least one intermediate image is generated in the object light path.
 11. The measuring device as recited in claim 9 or 10, wherein both the radiation leading to the object (O) to be measured and the radiation coming back from it pass through the optical probe (OS).
 12. The measuring device as recited in one of claims 7 through 11, wherein the reference mirror (RSP2) is provided on a flat plate or a prism.
 13. The measuring device as recited in one of claims 6 through 12, wherein fiber optics (LF) are situated between the beam splitter (ST1) and the additional beam splitter (ST2).
 14. The measuring device as recited in one of claims 9 through 13, wherein an endoscope is situated in the object light path for generating the intermediate image.
 15. The measuring device as recited in one of claims 3 through 14, wherein panorama optics or superpositioning optics are situated in the object light path.
 16. The measuring device as recited in one of claims 3 through 15, wherein it has several reference planes for scanning different measurement surfaces. 